Thrombosis, the development of blood clots within arterial vessels, is due to a complex mechanism involving the activation of both platelet aggregation and the coagulation protease cascade (Ann. Intern Med. (2001) 134: 224-38; N. Engl. J. Med. (2002) 347: 5-12; Thromb. Haemost. (2002) 86: 51-6). The pathways involved normally inhibit blood loss after vessel injury, but in thrombosis and related conditions, these reactions are inappropriately initiated and propagated.
On the molecular level, thrombosis is initiated by the release of mediators such as tissue factor (TF), von Willebrand Factor (vWF) (J. Thromb. Haemost. (2003) 1: 1602-12), and collagen from ruptured atherosclerotic plaques or from damaged blood vessels. Collagen and vWF bind to receptors on platelets and initiate their activation. Once activated, platelets release secretory granules containing ADP, ATP, and calcium (Curr. Opin. Hematol. (2001) 8: 270-6). Activated platelets also synthesize and release thromboxane. The released ADP and thromboxane bind to receptors on the platelets to further propagate platelet activation. Once platelets are activated they start aggregating to initiate clot formation.
TF and vWF also initiate the blood coagulation cascade, which consists of two separate pathways that converge on a common endpoint. Both pathways involve the serial activation of the serine protease clotting factors and ultimately lead to the activation of thrombin. Thrombin, once activated, cleaves fibrinogen to form fibrin. Thrombin, Factor Xa, and Factor VIIa can also activate platelets by cleaving the G protein-coupled protease-activated receptors PAR-1, PAR-3, and PAR-4 (Chest (2003) 124: 18S-25S). PAR-1, the prototype receptor, is activated following cleavage of its amino-terminal exodomain to produce a new amino-terminus (Cell (1991) 64: 1057-68). The new amino terminus then binds to the receptor to effect signaling (J. Biol. Chem. (1994) 269: 16041-45). PARs are therefore peptide receptors that contain their own ligand. PAR-2 is activated by trypsin and not by thrombin (Proc. Natl. Acad. Sci. USA (1994) 91: 9208-12).
Therefore, there is a need for compounds that inhibit the proteases of the blood and thus block platelet aggregation.